U.S. patent application number 09/725036 was filed with the patent office on 2001-08-02 for method of making optical fiber.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Kuwahara, Kazuya, Nagayama, Katsuya, Onishi, Masashi, Takamizawa, Hiroshi, Tsuchiya, Ichiro.
Application Number | 20010010162 09/725036 |
Document ID | / |
Family ID | 18312428 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010010162 |
Kind Code |
A1 |
Kuwahara, Kazuya ; et
al. |
August 2, 2001 |
Method of making optical fiber
Abstract
When drawing an optical fiber from a preform, positive and
negative dispersion sections of an optical fiber are provided,
respectively, with target glass diameters different from each
other, whereby the positive and negative dispersion sections attain
their respective desirable values of chromatic dispersion. Also,
the positive and negative dispersion sections and a transient
section therebetween are provided, respectively, with values of
control constant different from each other, which are used to
converge the measured glass diameter to said target diameter, for
adjusting the drawing speed, whereby the transient section is
shortened. Alternatively, the positive and negative dispersion
sections of the optical fiber are provided with respective target
drawing speeds different from each other, whereby the transient
section is shortened.
Inventors: |
Kuwahara, Kazuya;
(Yokohama-shi, JP) ; Tsuchiya, Ichiro;
(Yokohama-shi, JP) ; Nagayama, Katsuya;
(Yokohama-shi, JP) ; Onishi, Masashi;
(Yokohama-shi, JP) ; Takamizawa, Hiroshi;
(Yokohama-shi, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
Sumitomo Electric Industries,
Ltd.
|
Family ID: |
18312428 |
Appl. No.: |
09/725036 |
Filed: |
November 29, 2000 |
Current U.S.
Class: |
65/381 ; 65/378;
65/382; 65/402 |
Current CPC
Class: |
G02B 6/03627 20130101;
C03B 2203/36 20130101; C03B 37/0253 20130101; C03B 2203/06
20130101; C03B 2205/40 20130101; C03B 2203/18 20130101; G02B
6/02247 20130101 |
Class at
Publication: |
65/381 ; 65/378;
65/382; 65/402 |
International
Class: |
C03B 037/07; C03B
037/075 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 1999 |
JP |
P1999-337835 |
Claims
What is claimed is:
1. A method of making an optical fiber provided with positive and
negative dispersion sections having positive and negative chromatic
dispersions at a predetermined wavelength, respectively, which
sections are disposed alternately in a longitudinal direction of
the optical fiber drawn from a preform; wherein, when drawing the
optical fiber, said positive and negative dispersion sections of
said optical fiber are provided, respectively, with target glass
diameters different from each other, and said positive and negative
dispersion sections and a transient section therebetween are
provided, respectively, with values of control constant different
from each other, which are used to converge the measured glass
diameter to said target diameter, for adjusting a drawing
speed.
2. A method of making an optical fiber provided with positive and
negative dispersion sections having positive and negative chromatic
dispersions at a predetermined wavelength, respectively, which
sections are disposed alternately in a longitudinal direction of
the optical fiber drawn from a preform; wherein, when drawing the
optical fiber, said positive and negative dispersion sections of
said optical fiber are provided with respective target glass
diameters and target drawing speeds different from each other.
3. A method of making an optical fiber according to claim 2,
wherein, letting D.sub.1 and V.sub.1 be the target glass diameter
and target drawing speed in said positive dispersion section,
respectively, and D.sub.2 and V.sub.2 be the target glass diameter
and target drawing speed in said negative dispersion section,
respectively, the respective target glass diameters and target
drawing speeds of said positive and negative dispersion sections of
said optical fiber are set so as to satisfy the relational
expression of D.sub.1.sup.2.times.V.sub.1=D.sub.2.sup.2.times.-
V.sub.2.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of making an
optical fiber provided with positive and negative dispersion
sections having positive and negative chromatic dispersions at a
predetermined wavelength, respectively, which sections are disposed
alternately in the longitudinal direction of the optical fiber by
drawing an optical fiber preform.
[0003] 2. Related Background Art
[0004] It has been assumed that an optical fiber provided with
positive and negative dispersion sections having positive and
negative chromatic dispersions at a predetermined wavelength,
respectively, which sections are disposed alternately in the
longitudinal direction of the optical fiber for dispersion
management, can suppress both of the waveform deterioration caused
by nonlinear optical phenomena and that caused by cumulative
chromatic dispersion, thereby being suitably usable as an optical
transmission line of a wavelength division multiplexing (WDM)
transmission systems (see, for example, JP 8-320419A).
[0005] In general, when making an optical fiber from an optical
fiber preform, the lower end of the optical fiber preform is heated
and melted in a drawing furnace, and this optical fiber preform is
drawn, so as to produce the optical fiber. For making an optical
fiber such as the one mentioned above, a special step is provided
in order to change its chromatic dispersion in the longitudinal
direction thereof.
[0006] For example, the above-mentioned JP 8-320419A discloses a
technique in which an optical fiber preform whose core diameter or
preform diameter changes in the longitudinal direction thereof is
prepared, and the glass diameter is made constant when drawing an
optical fiber from the preform, so as to longitudinally change the
core diameter, thereby making an optical fiber whose chromatic
dispersion changes in the longitudinal direction thereof. Also, it
discloses a technique in which an optical fiber preform whose
refractive index profile and preform diameter are longitudinally
constant is prepared, and the glass diameter is changed when
drawing an optical fiber from the preform, so as to change the core
diameter, or the drawing tension is changed, so as to change the
refractive index according to the residual stress, thereby making
an optical fiber whose chromatic dispersion changes in the
longitudinal direction thereof.
[0007] On the other hand, JP 11-30725A discloses a technique in
which an optical fiber preform whose refractive index profile and
preform diameter are constant in the longitudinal direction thereof
is prepared, and the glass diameter is changed when drawing an
optical fiber from the preform, so as to change the core diameter,
thereby making an optical fiber whose chromatic dispersion changes
in the longitudinal direction thereof.
SUMMARY OF THE INVENTION
[0008] However, the above-mentioned conventional optical fiber
making techniques have problems as follows. Namely, when an optical
fiber is drawn at constant speed and constant tension from a
preform whose core diameter and preform diameter longitudinally
changes, the region where the diameter of preform changes becomes
so long after drawing. When changing the fiber diameter upon
drawing, a certain length of the optical fiber has to be drawn
until a desirable value is attained. Thus, in any of the
above-mentioned cases, a given length of the optical fiber where
chromatic dispersion changes exists between positive and negative
dispersion sections. This means that a given length of transient
section on the chromatic dispersion between positive and negative
dispersion sections exists in dispersion management optical fibers
thus obtained. Since the absolute value of chromatic dispersion is
small in this transient section, the effect of restraining the
waveform from deteriorating due to nonlinear optical phenomena
cannot fully be achieved.
[0009] In order to overcome the above-mentioned problems, it is an
object of the present invention to provide a method of making an
optical fiber which can easily make, with excellent
controllability, an optical fiber whose chromatic dispersion at a
predetermined wavelength changes in the longitudinal direction
thereof.
[0010] The present invention provides a method of making an optical
fiber provided with positive and negative dispersion sections
having positive and negative chromatic dispersions at a
predetermined wavelength, respectively, which are disposed
alternately in the longitudinal direction of the optical fiber
drawn from a preform; wherein, when drawing the optical fiber, the
positive and negative dispersion sections of the optical fiber are
provided, respectively, with target glass diameters different from
each other, and the positive and negative dispersion sections and a
transient section therebetween are provided, respectively, with
values of control constant different from each other, which are
used to converge the measured glass diameter to its target value,
for adjusting a drawing speed.
[0011] According to this optical fiber making method, when drawing
an optical fiber form a preform, the positive and negative
dispersion sections of the optical fiber are provided,
respectively, with target glass diameters different from each
other, whereby the positive and negative dispersion sections can
attain their respective desirable values of chromatic dispersion.
Also, as the control constant based on glass diameter for adjusting
the drawing speed is varied in the transient section between the
positive and negative dispersion sections, so that deviations
between the actual glass diameter and target glass diameters can be
eliminated rapidly, whereby the transient section can be shortened.
Namely, an optical fiber whose chromatic dispersion changes in the
longitudinal direction thereof can be made with a favorable
controllability according to this optical fiber making method. That
is, the transient section, where chromatic dispersion changes,
between the positive and negative dispersion sections can be
shortened, whereby the optical fiber made thereby can fully attain
the effect of restraining the waveform from deteriorating being due
to nonlinear optical phenomena.
[0012] Also, the present invention provides a method of making an
optical fiber; wherein, when drawing the optical fiber, the
positive and negative dispersion sections of the optical fiber are
provided with respective target glass diameters and target drawing
speeds different from each other.
[0013] According to this optical fiber making method, the positive
and negative dispersion sections of the optical fiber are provided,
respectively, with target glass diameters different from each
other, whereby the positive and negative dispersion sections can
attain their respective desirable values of chromatic dispersion.
Also, the positive and negative dispersion sections of the optical
fiber are provided, respectively, with target drawing speeds
different from each other, whereby the optical fiber can securely
attain a desirable glass diameter in each section, and the
transient section can be shortened. Namely, according to this
optical fiber making method, an optical fiber whose chromatic
dispersion changes in the longitudinal direction thereof can be
made with a favorable controllability, and the transient section,
where chromatic dispersion changes, between the positive and
negative dispersion sections can be shortened, whereby the optical
fiber made thereby can fully attain the effect of restraining the
waveform from deteriorating being due to nonlinear optical
phenomena.
[0014] Preferably, in this optical fiber making method, letting
D.sub.1 and V.sub.1 be the target glass diameter and target drawing
speed in the positive dispersion section, respectively, and D.sub.2
and V.sub.2 be the target glass diameter and target drawing speed
in the negative dispersion section, respectively, the respective
target glass diameters and target drawing speeds of the positive
and negative dispersion sections of the optical fiber are set so as
to satisfy the relational expression of
D.sub.1.sup.2.times.V.sub.1=D.sub.2.sup.2.times.V.sub.2. In this
case, only the target values are changed. The actual drawing speed
differs from its target value by an amount of correction caused by
control, nevertheless, it is preferred that this amount of
correction is not returned to zero upon changing the target values.
It is theoretically desirable to multiply the amount of correction
by a coefficient, but practically, it is not necessary to multiply
by coefficient within the range of the target values.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a view explaining an example of diameter changes
in the optical fiber made by the method of making an optical fiber
in accordance with the present invention;
[0016] FIG. 2 is a view explaining an example of refractive index
profile of the optical fiber made by the method of making an
optical fiber in accordance with the present invention;
[0017] FIG. 3 is a graph showing chromatic dispersion
characteristics of optical fibers having the refractive index
profile shown in FIG. 2;
[0018] FIG. 4 is a schematic explanatory view of a manufacturing
apparatus for carrying out the method of making an optical fiber in
accordance with the present invention;
[0019] FIG. 5 is a block diagram explaining the drawing speed
control for the optical fiber in the method of making an optical
fiber in accordance with the present invention;
[0020] FIG. 6 is a block diagram explaining the feeder speed
control for the optical fiber preform in the method of making an
optical fiber in accordance with the present invention;
[0021] FIG. 7 is a graph showing changes in drawing speed with time
in examples and a comparative example as compared with each other;
and
[0022] FIG. 8 is a graph showing longitudinal glass diameter
distributions of optical fibers made in the examples and
comparative example as compared with each other.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] In the following, embodiments of the present invention will
be explained in detail with reference to the accompanying drawings.
To facilitate the comprehension of the explanation, the same
reference numerals denote the same parts, where possible,
throughout the drawings, and a repeated explanation will be
omitted.
[0024] First, an example of optical fiber made by the method of
making an optical fiber in accordance with the present invention
will be explained with reference to FIG. 1. In the optical fiber 10
shown in this drawing, positive dispersion sections 11 having a
positive chromatic dispersion at a predetermined wavelength (e.g.,
a wavelength of 1.55 .mu.m) and negative dispersion sections 12
having a negative chromatic dispersion at the predetermined
wavelength are disposed alternately in the longitudinal direction
of the optical fiber 10, whereby dispersion management is effected.
By holding the absolute value of local chromatic dispersion high
(e.g., so as to become at least 1 ps/nm/km) in most of regions, the
optical fiber 10 can restrain the waveform from deteriorating being
due to nonlinear optical phenomena. Also, by lowering the average
chromatic dispersion over the whole length, the optical fiber 10
can restrain the waveform from deteriorating being due to
cumulative chromatic dispersion. Therefore, the optical fiber 10 is
suitably used as an optical transmission line of a WDM transmission
system. In the optical fiber 10, the positive dispersion section 11
and negative dispersion section 12 are provided with glass
diameters and core diameters different from each other. There are
transient section, where the diameter changes, between the positive
dispersion section 11 and the negative dispersion section 12
[0025] FIG. 2 is an explanatory view of an example of refractive
index profile of the optical fiber 10. FIG. 3 is a graph showing
chromatic dispersion characteristics of optical fibers having the
refractive index profile shown in FIG. 2. The optical fiber 10
comprises, successively from its center, a core region having a
maximum refractive index n.sub.1 and an outside diameter 2a, a
depressed region having a refractive index n.sub.2 and an outside
diameter 2b, and a cladding region having a refractive index
n.sub.3, whereas the individual refractive indices have a
relationship of n.sub.1>n.sub.3>n.sub.2 in terms of
magnitude. Such a refractive index profile can be realized, for
example, when the core region and the depressed region consist of
GeO.sub.2-doped SiO.sub.2 and F-doped SiO.sub.2 respectively, while
the cladding region consists of SiO.sub.2.
[0026] The graph shown in FIG. 3 indicates the wavelength
dependence of chromatic dispersion with respect to each value
(10.45 .mu.m to 11.65 .mu.m) of outside diameter 2b of the
depressed region. Here, with reference to the refractive index of
cladding region, the relative refractive index difference of core
region .DELTA..sub.1=n.sub.1-n.sub.3 is set to 0.9%, the relative
refractive index difference of depressed region
.DELTA..sub.2=n.sub.2-n.sub.3 is set to -0.45%, and the ratio
(2a/2b) of the respective outside diameters of core region and
depressed region is set to 0.58. As shown in this graph, the
chromatic dispersion at a wavelength of 1.55 .mu.m becomes greater
as the outside diameter 2b of depressed region increases, i.e., the
outside diameter 2a of core region increases.
[0027] In the optical fiber 10 explained in FIGS. 1 to 3, each
positive dispersion section 11 has a glass diameter of 131 .mu.m
and a chromatic dispersion of +3 ps/nm/km. Each negative dispersion
section 12 has a glass diameter of 123 .mu.m and a chromatic
dispersion of -3 ps/nm/km.
[0028] An outline of the method of making an optical fiber will now
be explained with reference to FIG. 4. Characteristic features of
the method of making an optical fiber in accordance with the
present invention will be explained later. This drawing
schematically shows an optical fiber manufacturing apparatus, in
which control sections are depicted as a block diagram.
[0029] In this optical fiber manufacturing apparatus, an optical
fiber preform 20 is attached to a feeder 110 and is set within a
drawing furnace 120. Then, the lower end of the optical fiber
preform 20 is heated and melted by the heater of the drawing
furnace 120, so as to form a neck-down part, and the optical fiber
10 is drawn from thus heated and melted lower end of optical fiber
preform 20. The optical fiber 10 coming out of the drawing furnace
120 is, with its glass diameter being measured by a glass diameter
meter 130, forcibly cooled by cooling unit (not depicted). The
result of measurement effected by the glass diameter meter 130 is
reported to a glass diameter control section 310.
[0030] Thereafter, the optical fiber 10 passes through a resin
coating die 140, so as to be coated with a resin, which
subsequently is cured by ultraviolet light irradiation with a UV
lamp 150, whereby the optical fiber 10 is covered with a primary
coating. Further, the optical fiber 10 passes through a resin
coating die 160, so as to be coated with a resin, which
subsequently is cured by ultraviolet light irradiation with a UV
lamp 170, whereby the optical fiber 10 is covered with a secondary
coating. The optical fiber 10 thus covered with the resin coatings
is taken up by a bobbin 241 of a take-up 240 by way of a roller
210, a capstan 221 of a fiber puller 220, and a dancer section 230
in succession.
[0031] Based on a set drawing speed, the glass diameter control
section 310 controls the drawing speed of the optical fiber 10 in
the fiber puller 220 such that the glass diameter of optical fiber
10 measured by the glass diameter meter 130 converges to its target
values. Based on a feeder speed calculated from the target drawing
speed, preform diameter, and target glass diameter, a drawing speed
control section 320 controls the feeder speed of the preform
feeding unit 111. In response to a command from the drawing speed
control section 320 and a feeder speed calculating section, the
preform feeding unit 111 drives the feeder 110, so as to insert the
optical fiber preform 20 into the drawing furnace 120. In response
to a command from the glass diameter control section 310 and the
set drawing speed, the fiber puller 220 drives the capstan 221, so
as to set the drawing speed of optical fiber 10.
[0032] The glass diameter control and drawing speed control
effected by the glass diameter control section 310 and drawing
speed control section 320 will now be explained. FIG. 5 is a block
diagram explaining the drawing speed control for the optical fiber
10. From the actually measured glass diameter value D.sub.g
measured by the glass diameter meter 130 and the target glass
diameter D.sub.s, a glass diameter deviation .DELTA.D is determined
by the following expression:
.DELTA.D=D.sub.g-D.sub.s (1)
[0033] According to this glass diameter deviation .DELTA.D, a
drawing speed control amount .DELTA.V.sub.1 is determined by the
following expression:
.DELTA.V.sub.1=.alpha..sub.1.times..DELTA.D+.beta..sub.1.times..intg..DELT-
A.Ddt+.gamma..sub.1.times..DELTA.D/dt (2)
[0034] From the drawing speed control amount .DELTA.V.sub.1 and a
target drawing speed V.sub.g, the drawing speed vf of optical fiber
10 is determined by the following expression:
V.sub.f=V.sub.g(1+.DELTA.V.sub.1) (3)
[0035] Then, based on the drawing speed V.sub.f, the rotational
speed of capstan 221 of the fiber puller 220 is controlled.
[0036] FIG. 6 is a block diagram explaining the feeder speed
control for the optical fiber preform 20. According to the preform
outside diameter D.sub.p of optical fiber preform 20, the target
glass diameter D.sub.s of optical fiber 10, and the target drawing
speed V.sub.g of optical fiber 10, the target feeder speed V.sub.s
of optical fiber preform 20 is determined by the following
expression:
V.sub.s=(D.sub.s/D.sub.p).sup.2.times.V.sub.g (4)
[0037] On the other hand, according to the target drawing speed
V.sub.g and actual drawing speed V.sub.f of optical fiber 10, a
drawing speed deviation .DELTA.V.sub.2 is determined by the
following expression:
.DELTA.V.sub.2=V.sub.f-V.sub.g (5)
[0038] According to this drawing speed deviation .DELTA.V.sub.2,
the feeder speed control amount .DELTA.V.sub.3 of optical fiber
preform 20 is determined by the following expression:
.DELTA.V.sub.3=(D.sub.s/D.sub.p).sup.2.times..left
brkt-bot..alpha..sub.2.-
times..DELTA.V.sub.2+.beta..sub.2.times..intg..DELTA.V.sub.2dt+.gamma..sub-
.2.times..DELTA.V.sub.2/dt.right brkt-bot. (6)
[0039] According to the set feeder speed V.sub.s and feeder speed
control amount .DELTA.V.sub.3 of optical fiber preform 20, the
feeder speed V.sub.p of optical fiber preform 20 is determined by
the following expression:
V.sub.p=V.sub.s-.DELTA.V.sub.3 (7)
[0040] Then, according to the feeder speed V.sub.p, the feeding
speed of optical fiber preform 20 generated by the preform feeding
unit 111 is controlled.
[0041] In practice, both the capstan motor and preform feeder motor
employ servo motors, whereas respective motor drivers carry out
appropriate control therewithin in order to stably realize drawing
speeds V.sub.g+.DELTA.V.sub.1, V.sub.s-.DELTA.V.sub.3 to be
inputted. It will be sufficient if recommended values for motor
meters are used. For example, values of the control constants
appearing in the above-mentioned expressions (2) and (6) are:
.alpha..sub.1=5.times.10.sup.2 [/m] (8a)
.beta..sub.1=5.times.10.sup.4 [/m.multidot.s] (8b)
.gamma..sub.1=0 [s/m] (8c)
.alpha..sub.2=0.75 [-] (8d)
.beta..sub.2=0.005 [/s] (8e)
.gamma..sub.2=0 [s] (8f)
[0042] The drawing speed control for the optical fiber and the
feeder speed control for the optical fiber preform are coupled with
each other at the drawing speed V.sub.f of optical fiber 10, thus
failing to form independent control systems. However, while the
glass diameter control responds in seconds, the drawing speed
control responds in minutes since it accompanies changes in the
melting state of optical fiber preform 20. As a consequence, the
time constant greatly differs between the respective control
systems for glass diameter control and drawing speed control,
whereby the two control systems can be treated as substantially
independent systems.
[0043] The characteristic features of a first embodiment of the
method of making an optical fiber in accordance with the present
invention, which will be explained in detail in the following,
reside in that, in the optical fiber making method explained in the
foregoing, the positive dispersion section 11 and negative
dispersion section 12 of the optical fiber 10 are provided,
respectively, with target glass diameters different from each
other, and the positive dispersion section 11, negative dispersion
section 12, and transient section are provided with their
respective different values of control constant based on glass
diameter for the drawing speed in expression (2). A second
embodiment is characterized in that the positive dispersion section
11 and negative dispersion section 12 of the optical fiber 10 are
provided with target glass diameters different from each other and
target drawing speeds different from each other. As a consequence,
the transient section between the positive dispersion section 11
and negative dispersion section 12 becomes shorter in the
dispersion management optical fiber 10, whereby the effect of
restraining the waveform from deteriorating due to nonlinear
optical phenomena can fully be achieved.
[0044] First Embodiment
[0045] To begin with, the first embodiment of the method of making
an optical fiber in accordance with the present invention will be
explained. In the first embodiment, the positive dispersion section
11 and negative dispersion section 12 of the optical fiber 10 are
provided, respectively, with target glass diameters D.sub.s
different from each other, whereas the positive dispersion section
11 and negative dispersion section 12, and transient section are
provided with their respective different values of control constant
based on glass diameter for the drawing speed.
[0046] In the following explanation, it is assumed that the
positive dispersion section 11 is drawn at a constant glass
diameter during a period from time t.sub.1 to time t.sub.2, the
target glass diameter is changed at time t.sub.2, the actually
measured glass diameter value converges to the target glass
diameter at time t.sub.3, and the negative dispersion section 12 is
drawn at a constant glass diameter during a period from time
t.sub.3 to time t.sub.4.
[0047] During the period (from time t.sub.1 to time t.sub.2) when
the positive dispersion section 11 is drawn while the target glass
diameter D.sub.s is set at a constant diameter D.sub.1, the drawing
speed control amount .DELTA.V.sub.1 is determined by the
above-mentioned expression (2). Also, the drawing speed control
amount .DELTA.V.sub.1 is determined by the above-mentioned
expression (2) during the period (from time t.sub.3 to time
t.sub.4) when the negative dispersion section 12 is drawn while the
target glass diameter D.sub.s is set at a constant diameter
D.sub.2. During the period (from time t.sub.2 to time t.sub.3)
drawing the transient section where the actually measured glass
diameter value D.sub.g changes between the positive dispersion
section 11 and negative dispersion section 12, however, the drawing
speed control amount .DELTA.V.sub.1 is determined by the following
expression:
.DELTA.V.sub.1=2.times..alpha..sub.1.times..DELTA.D+2.times..beta..sub.1.t-
imes..intg..DELTA.Ddt+.gamma..sub.1.times..DELTA.D/dt (9)
[0048] Thus, the control constants (2.alpha..sub.1,
2.times..beta..sub.1) for the drawing speed in the transient
section are two times the respective control constants
(.alpha..sub.1, .beta..sub.1) in the positive dispersion section 11
and negative dispersion section 12, and are different from each
other. As for .beta..sub.1, however, only its part during the
period from time t.sub.2 to time t.sub.3 is changed. This is
possible without any problems in current digital processing control
systems.
[0049] In this case, if the target glass diameter was changed while
the target drawing speed V.sub.g was kept at a constant value of
500 m/min, and the drawing speed control constant of transient
section was twice that of each of the positive dispersion section
11 and negative dispersion section 12, then it took 60 m as the
length of optical fiber 10 until the actually measured glass
diameter value converged to the target glass diameter from the time
when the target glass diameter was altered. In this embodiment,
since the volume flow rate of glass drawn from the preform would
change between before and after the target glass diameter was
altered, it took a relatively long period of time until the glass
diameter and drawing speed became stable after the setting was
altered. Also, since the preform feeder speed changed due to
actions of the drawing speed control system, the drawing speed of
optical fiber gradually approached the target drawing speed of
V.sub.g=500 m/min after once dropped greatly as shown in FIG.
7.
[0050] When control was carried out according to the
above-mentioned expression (9) immediately after starting the
drawing, oscillation was likely occur upon the glass diameter and
drawing speed at the time of initial low drawing speed, whereby the
drawing speed was hard to increase. Therefore, it is desirable that
the drawing speed control constants be changed when actually
altering the glass diameter after a stable drawing speed for
production is attained.
[0051] Second Embodiment
[0052] The second embodiment of the method of making an optical
fiber in accordance with the present invention will now be
explained. In the second embodiment, the positive dispersion
section 11 and negative dispersion section 12 of the optical fiber
10 are provided with respective target glass diameters D.sub.s
varied each other and respective target drawing speeds V.sub.g
varied each other.
[0053] More preferably, let D.sub.1 and V.sub.1 be the target glass
diameter D.sub.s and target drawing speed V.sub.g in the positive
dispersion section 11, respectively, and D.sub.2 and V.sub.2 be the
target glass diameter D.sub.s and target drawing speed V.sub.g in
the negative dispersion section 12, respectively. Then, the
respective target glass diameters and target drawing speeds of the
positive dispersion section 11 and negative dispersion section 12
of optical fiber 10 are set so as to satisfy the following
relational expression:
D.sub.1.sup.2.times.V.sub.1=D.sub.2.sup.2.times.V.sub.2 (10)
[0054] Here, such an operation as .DELTA.V.sub.1 and .DELTA.V.sub.3
are set to be zero is not carried out. Also, none of .alpha..sub.1,
.beta..sub.1, .gamma..sub.1, .alpha..sub.2, .beta..sub.2, and
.gamma..sub.2 is altered. As V.sub.g is altered, the amount of
control V.sub.g.DELTA.V.sub.1 slightly changes, but its effect is
small. This is theoretically superior in terms of continuity.
[0055] For example, let the target glass diameter D.sub.1 and
target drawing speed V.sub.1 in the positive dispersion section 11
be 131 .mu.m and 440.8 m/min, respectively, and the target glass
diameter D.sub.2 and target drawing speed V.sub.2 in the negative
dispersion section 12 be 123 .mu.m and 500 m/min, respectively. In
this case, it took 12 m as the length of optical fiber 10 until the
actually measured glass diameter value converged to the target
glass diameter from the time when the target glass diameter was
altered.
[0056] Comparative Example
[0057] The optical fiber making method of a comparative example
will now be explained. In this comparative example, only the target
glass diameter D.sub.s is varied at the positions between the
positive dispersion section 11 and negative dispersion section 12
of the optical fiber 10. Namely, the drawing speed control constant
and the target drawing speed V.sub.g are kept at a constant.
[0058] In this case, if the target glass diameter was changed while
the target drawing speed V.sub.g was kept at a constant value of
500 m/min, then it took 100 m as the length of optical fiber 10
until the actually measured glass diameter value converged to its
target value from the time when the target glass diameter was
altered. In this comparative example, since the volume flow rate of
glass drawn from the preform changed if the target glass diameter
was changed, the drawing speed of optical fiber 10 fluctuated
during a relatively long period of time. Due to actions of the
drawing speed control system, however, the drawing speed of optical
fiber 10 gradually approached the target drawing speed of
V.sub.g=500 m/min.
[0059] Comparison of First and Second Embodiments and Comparative
Example The respective optical fiber making methods of the first
and second embodiments and comparative example will now be compared
with each other.
[0060] FIG. 7 is a graph showing the change of drawing speed with
time. This graph is based on the time when the target glass
diameter is altered. As can be seen from this graph, the drawing
speed of optical fiber 10 in the comparative example started
changing from the time when the target glass diameter was altered,
and attained its minimum value about 12 seconds thereafter. Then,
the drawing speed started to approach its original value of 500
m/min, and further approached 500 m/min though not depicted. In the
first embodiment, the drawing speed of optical fiber 10 started
changing from the time when the target glass diameter was altered,
and attained its minimum value about 4 seconds thereafter. Then,
the drawing speed started to approach its original value of 500
m/min, and further approached 500 m/min though not depicted. In the
second embodiment, the drawing speed became stable at a new setting
value 440.8 m/min after 2 seconds from the time when the target
glass diameter was changed.
[0061] FIG. 8 is a graph showing the longitudinal glass diameter
distribution of optical fiber 10. This graph is also based on the
time when the target glass diameter is changed. As can be seen from
this graph, the length of optical fiber 10 needed for the actually
measured glass diameter value to converge to its target value is
100 m, 60 m, and 12 m in the comparative example, first embodiment,
and second embodiment, respectively.
[0062] As in the foregoing, the time required for the drawing speed
to become stable and the fiber length required for the actually
measured glass diameter value to converge to its target value are
shorter in the first and second embodiments than in the comparative
example. Also, the time required for the drawing speed to become
stable and the fiber length required for the actually measured
glass diameter value to converge to its target value are shorter
and the actually measured glass diameter value converges more
stably in the second embodiment than in the first embodiment.
[0063] Therefore, according to the respective optical fiber making
methods in accordance with the first and second embodiments, the
optical fiber 10 whose chromatic dispersion changes in the
longitudinal direction thereof can be made with a favorable
controllability, and the transient section where chromatic
dispersion changes between the positive and negative dispersion
sections can be shortened. Hence, the optical fiber 10 made thereby
can fully achieve the effect of restraining the waveform from
deteriorating due to nonlinear optical phenomena. Such an effect is
more remarkable in the second embodiment than in the first
embodiment.
* * * * *